Many switching systems currently available contain a conference call feature which allows three or more stations to be connected together simultaneously so that each can communicate with all of the others. Various techniques for affecting such an arrangement are well known and are described in some detail in U.S. Pat. No. 4,224,688. In general, some means must be provided for combining the several signals so that each conferee can listen to all the other conferees. Typically, this is done by adding the signals from all the conferees and then subtracting that of the listening station. In an analog system this is relatively easy to do. It becomes more difficult in a digital system and, in particular, in nonlinear digital systems.
Alternative approaches involve sampling all the signals and transmitting only the largest one, or scanning to determine which party is speaking. All of these techniques require a considerable investment in equipment which one would prefer to avoid.